The type of fastener driving device herein contemplated typically includes a fastener driving element or driver which is mounted within a drive track within which successive fasteners are fed. The driver is moved through repetitive cycles, each of which includes a drive stroke during which the fastener is moved out of the drive track into the workpiece and a return stroke. The fastener driving element is fixedly connected with a piston which is mounted within a cylinder for movement through a drive stroke and return stroke with the fastener driving element. The piston is driven by gases under pressure preferably in the form of compressed air, applied to an operative surface area of the piston. While the invention is particularly suited to compressed air units, there can be applicability to internal combustion and ballistic actuated units where relatively low operating temperatures are contemplated.
The driver-piston assembly of a device of this type is inherently subjected to high stress forces tending to produce wear on the driver and disassembly of the driver from the piston. Due to the latter tendency, the usual connection between the fastener driving element and the piston constitutes an interengaging thread connection. This connection may take several forms. One form is to provide a central through bore in the piston which is formed with internal threads and to provide on the fastener driving element a cylindrical end portion the inner end of which terminates in a radially extending annular flange. The cylindrical end portion of the fastener driving element is provided with exterior threads which mesh with the interior threads of the piston. The usual arrangement is to effect a threaded interengagement of the two parts with the use of a locking compound, such as Loctite, with the annular flange of the fastener driving element being torqued into engagement with the piston at the portion thereof below the lower end of the bore. In arrangements of this type, the piston and fastener driving elements are formed with flat surfaces so that in the final assembly appropriate tools can be used to apply an appropriate torque to the assembly. The torquing of the intermeshing relationship between the threads which creates substantial opposing axial forces in the intermeshing threads adds to the efficacy of the joint enabling the length of meshing interengagement to be minimized. On the other hand, the need to provide the flange which permits a high torque to be applied, materially increases the cost of the fastener driving element.
While torqued assemblies with flanged drivers are most prevalent, there are units which are neither flanged nor torqued. An example of such a unit is included in a device identified as Hitachi Model NR83A. The Hitachi Model NR 83A includes a piston and fastener driving element assembly in which the fastener driving element includes exterior threads without a torquing flange and meshing interengagement with the interior threads of the piston is accomplished without creating substantial opposing axial forces between the threads. A locking compound is used to set the final assembly. The arrangement does not contemplate convenient disassembly since, with the elimination of the need to torque to the assembly, the usual tool engaging flat surfaces provided for the purpose of torquing have been eliminated.
Another type of assembly involves the extension of the cylindrical end of the fastener driving element to a point beyond the threaded bore and then providing a locking nut on the extended end which serves to engage the portion of the piston surrounding the upper end of the bore. With an arrangement of this type, it is possible to eliminate the use of the locking compound if the nut is of the locking type, as, for example, a lock nut or the like. A disadvantage of the second type of connection wherein a lock nut is utilized is that the arrangement tends to extend the height of the tool and to add weight or increase the mass of the driver-piston combination.
In situations where the tools are used extensively, a driver can show sufficient wear as to require replacement for as many as three or more times during the life of the tool. The driver is a fairly expensive part which must be hardened by heat treatment or the like. The usual practice is to replace the driver in the assembly by removing the same from the piston and reconstructing the connection with a new driver. The wear on the driver that requires replacing is wear on the lower fastener engaging surface thereof. Where this wear gets to an extent equal to one-eighth of an inch, replacement is in order. Moreover, it will be understood that variation in the length of the driver to an extent of one-eighth of an inch can result in a significant difference in the extent to which the head of the fastener is countersunk with respect to the workpiece. It is also sometimes the practice to recondition worn drivers by grinding the lower end to provide a new fastener engaging surface and to utilize shims when the reconditioned driver is reconnected with the piston. In all of the situations outlined above, it is necessary to effect an initial disconnection between the piston and the old driver and then to effect a reconnection between the piston and either a new driver or a reconditioned driver. In the case where final locking is accomplished by a locking compound rather than a locking nut, new locking compound must be applied as the reconnection takes place.